Petrochemical offgases, such as refinery offgases from Fluid Catalytic Cracker Units (FCCU) or Coker units, generally comprise an offgas mixture which comprises: hydrogen; nitrogen; carbon monoxide; ethane; ethylene; argon; propylene; as well as, butane and pentanes. In particular, the refinery offgasses from a FCCU contain olefin components, up to about 20 percent by volume ethylene and up to about 11 percent by volume propylene, which components normally are not recovered from the offgases, but which components may have value to warrant recovery and use in other petrochemical processes or uses in downstream processing. The typical range of components is listed in Table 1.
Normally, such refinery offgases are burnt and used as fuel. When the refinery FCCU is located near an ethylene or propylene plant, then recovery and downstream use of the recovered ethylene and propylene or propylene is economical. In such recovery processes, ethylene and propylene are recovered together and then directly or separately recovered from the mixture.
In the publication, Chemical Engineering, May 1999, pages 30-33, "Refiners Get Cracking on Petrochemicals" hereby incorporated by reference, the attributes of Fluid Catalytic Cracker (FCC) technology are explained. Propylene is increasing in demand, since it is a feedstock for polypropylene. About 30 percent (30%) of the global demand is a byproduct of FCC operations for producing gasoline. Over the next five years, the growth in demand is anticipated at over 5 percent (5%) per year. In response to this demand, FCC process technology is being modified to enhance olefin's production. Thus, the current yield of 5 percent (5%) of the feedstock converted to propylene is being increased to about 35 percent (35%), while the ethylene yield is increased from about 1 to 4 percent (1-4%).
Another option is the reaction of ethylene with 2-butene to produce propylene, termed metathesis. Thus, the recovery of these components from a FCC provides the opportunity for further propylene yields.
U.S. Pat. No. 5,546,764, issued Aug. 20, 1996, and incorporated herein by reference, discloses an absorption process for recovering ethylene from a feed gas stream in an absorber stripper employing a heavy hydrocarbon absorption solvent, the absorber stripper bottoms stream is then fractionated to produce an overhead, ethylene product stream.
The recovery of valued olefin components from offgasses may be accomplished by partial condensation from the offgas feed stream; however, such recovery requires low temperatures, e.g., about --150.degree. F. or lower. These low temperatures require refrigeration supplied by turboexpander processes and heat exchangers with multi-pass plate fin heat exchangers. This type of equipment is not used extensively in oil refineries. Furthermore, the use of such low temperatures increases the possibility of gum formation by the reaction of NOX compounds with butadiene, which gums may be explosive when the plant is shut down and warmed up for restart.
U.S. Pat. No. 5,345,772, issued Sept. 13, 1994, shows a single column process with Natural Gas Liquid (NGL) recycled to the upper zone of the column. This process is applied to the recovery of the paraffinic components of propane or ethane from gas streams with a high CO.sub.2 content. These gases are encountered in enhanced oil recovery projects where the CO.sub.2 content of the gas stream may be 83 percent (83%), as shown in the patent example.
It is desirable to provide a system and process for the recovery of olefins from offgases at warmer temperatures, such as propane refrigeration levels, to avoid the disadvantages of prior art recovery processes and systems and to provide other economic and process efficiency advantages.